docs: Update the PyMAPDL documentation page for MPREAD

[germa89]

@germa89 Well crap on a cracker...why did I not check the source code? Mmmmmm. I think we can just update the PyMAPDL documentation page for MPREAD so we know how the command works. I'm fine with finding (if needed) and uploading a material library file if I need the file.

Originally posted by @mikerife in https://github.com/ansys/pymapdl/discussions/3333#discussioncomment-10257388